In audio listening applications, it is normally desirable to minimize the amount of background noise heard by the user. Methods for achieving such reduction fall into two main categories, passive and active. Passive noise reduction is accomplished by acoustically isolating the listener from the external noise source through the use of insulation or other sound blocking materials. However, the results are often unsatisfactory due to the difficulty of effectively blocking frequencies in the lower range of the audible spectrum.
Active noise reduction systems use the principle of phase reversal to cancel out unwanted signals. In these systems, a microphone is used to sense external background noise. This signal is then phase shifted to create a cancellation signal and added to the intended audio program signal sent to the speaker. The cancellation signal combines with the noise and effectively reduces or eliminates the level of unwanted noise perceived by the listener.
One shortcoming to active noise cancellation systems currently available is that a dedicated microphone must be incorporated to sense the noise heard by the user. For example, noise cancelling headphone sets will typically employ one microphone per ear piece and have their own power supply which energizes an electronic circuit to process the signal from the microphones and generate the cancellation signal. This additional circuitry increases the size and cost of such units and limits their marketability to consumers. Additional problems are presented due to the distance between the sensing microphone, the speaker, and the listener's ear, making cancellation of higher frequency noise signals problematic.